Project Summary/Abstract Romantic relationships are one of the most salient social relationships for health during the transition to adulthood and into adulthood. Different-sex relationships that develop in the typical stage of life, and certain relationship structures, such as marriage, are linked to better physical and mental health for heterosexual individuals. Yet, research on romantic relationships fails to consider the relationship experiences of sexual minority adolescents and young adults, including lesbian, gay, or bisexual individuals. Sexual minorities face unique barriers to developing romantic relationships during the transition to adulthood that may offset any health benefits gained from these relationships. Because sexual minorities experience elevated substance use and mental health problems compared to their heterosexual counterparts, understanding the role of romantic relationships in contributing to sexual minority health is critical. The overarching goal of the proposed project is to establish how associations between sexual minority romantic relationships and health vary by relationship timing, structure, and quality using data from a large, nationally representative sample. To meet this goal, I will first identify how sexual minorities form a first dating relationship and romantic union, defined as cohabitation or marriage, and establish how this timing affects substance use and mental health in adulthood. This research will provide insight into whether, and how much, sexual minorities deviate from heterosexual individuals' well- established relationship trajectories, and, how any deviation affects health. Second, I will test how types of and changes in sexual minority relationship structures throughout the transition to, and into, adulthood contribute to substance use and mental health. Meeting this objective will establish which relationships structures are more vulnerable or resistant to substance use or mental health problems. Third, I will examine how relationship quality moderates the association between relationship structure and health for sexual minorities, which will provide insight into the characteristics of relationships most protective for health. The proposed project will enhance our knowledge about how sexual minority relationship timing and structure are either protective or detrimental for health from adolescence to adulthood, and identify how relationship-specific processes alter these pathways. The knowledge gained from this research will ground the development of future scholarship designed to explore additional mechanisms, such as the role of minority stress, through which sexual minority relationships contribute to, or reduce, health.